This invention generally relates to the art of organic synthesis and specifically to the production of o-nitrobenzaldehyde.
Ortho-nitrobenzaldehyde, also referred to herein as ONBA for short, has the formula (1) ##STR1## and one of the first methods for its production was disclosed as early as 1889 (German Pat. No. 48,722).
ONBA is a normally solid (commercially available ONBA melts at 42.degree.-44.degree. C.) pale-yellow substance that is almost insoluble in water but soluble in both polar and apolar organic solvents; it can be used, for example, as an indicator or reagent for isopropanol and acetone, as an intermediate product in the production of pharmaceutically active 4-(2-nitrophenyl)-1,4-dihydropyridines as disclosed, for example, in German published patent application (DE-OS) No. 16 70 827, and for other organic syntheses where a benzene ring with an aldehyde and nitro substituent is needed.
Yet, as recently as 1965-1968, ONBA has been said to be accessible with difficulties only (cf. Cassebaum, H., J. pr. Chem. Vol. 4, page 29 [1965] of Fieser, L. M. and M., Organische Chemie, Weinheim [1968], page 1004) and several new methods for producing ONBA have been suggested during the last decade.
A feature that is common to many ONBA syntheses including the most recent ones is to start from o-nitrotoluene which is contacted as such or in the form of a suitable derivative with an oxidizing agent; for example, use of chromic acid in acetic anhydride is suggested in Org. Syntheses, Coll. Vol. III, page 641, but large volumina are required and the product is obtained together with tarry by-products so as to require complicated recovery procedures.
Cassebaum (loc cit.) suggests to treat o-nitrotoluene with nitric acid for producing a nitrated intermediate (o-nitrophenyl nitromethane) which, in turn, is treated with potassium permanganate.
In an attempt to avoid the use of such oxidizing agents as potassium permanganate, one of the more recent synthesis methods disclosed, for example, in DE-OS No. 24 15 062 is based upon hydrolysis of o-nitrobenzylidene chloride ##STR2## which, in turn, is obtained by treating o-nitrophenyl pyruvic acid with aqueous alkali metal hypochlorite; thus, the immediate precursor compound of ONBA is o-nitrobenzylidene chloride in this method.
The use of o-nitrophenyl pyruvic acid compounds as immediate precursors in the synthesis of ONBA has been suggested in DE-OS No. 24 15 061; the detour via the o-nitrobenzylidene chloride is avoided in that method and a salt of o-nitrophenyl pyruvic acid and an alkali metal as the precursor compound is contacted with potassium permanganate: ##STR3## While the precursor or salt of formula (10) can be conveniently obtained from o-nitrotoluene by reaction with a lower alkyl diester of oxalic acid in the presence of a lower alkanolate, such as sodium or potassium methylate, the use of potassium permanganate as the oxidizing agent is required again and causes most of the disadvantages that are typical for permanganate; this includes separation of the target compound ONBA from the manganic compounds that are obtained as a result of the reduction of the MnO.sub.4.sup.- ion in the process, as well as waste water problems encountered with manganese compounds.
Thus, according to the art, no process of producing ONBA from the synthetically convenient o-nitrophenyl pyruvates is known where the target product may be obtained by treating the pyruvate precursor with an oxidizing agent that is more convenient than a permanganate.